I need case ideas for my headless iMac.

Are you thinking about mounting it in an everyday object???  If I remember correctly its a G4 (hence headless :P)  I would go to a thrift store and buy a big ugly lamp and keep the lamp functioning.  That would be hilarious. 
If you don't want funny... I've been thinking about a ceramic computer case for a while.  I've got a few friends who are potters and it could be really beautiful and unique. 

I just used an ATX computer case I can have 2 harddrives and a CD drive and it was cheap

I have a great one, but you'll need to do a little chemistry first.
I think you should make a Macquarium. But this one will be different. Instead of converting a dead Mac into a fishtank, convert a perfectly good fishtank into a working Mac.

You install the iMac parts into the bottom of the fishtank. You can add gravel around it a bit, as long as its electrically insulating gravel. You cut a hole, a little above the level of the highest part of the iMac components, and fit a tap (do you guys call these faucets when they are fitted to things other than sinks or washbasins?).
I would run the power cable up and out of the top of the fishtank for safety reasons.

Here comes the chemistry: What you need is an oily substance, immiscible with water, as transparent as possible, but most of all, denser than water (non-toxic would be good). You fill the tank to just below the tap, then on the top of the oil layer, you fill with water, plants and fish. When you want to change the water, remove the fish as you would anyway, then drain the water from the top layer, before replacing it and the fish.

Alternatively, you could use any old oil and keep it separate from the fishtank above. But thats just an oil-cooled iMac in a glass case with a fishtank on it. The first way has extra cool factor. Either way, you have an original mod and an oil cooled iMac to play with. You could probably overclock it a bit, and the water would be good for conducting extra heat away from the oil (especially in the direct contact version, or you could separate them with a layer of metal).
If you use it to run SETI, folding or similar, you might even get it to heat the tank. Then you can have tropical fish. Woo!

You read my mind i wanted to do that for ages. But i didnt no how.

Well I have seen mods where people water cool their systems by dumping the whole system into a fishtank and use distilled water or some non conductive goop but fish couldn't live in them. The only fish safe way is to make a case out of plastic that has a 3 to 5 inch cavity and fill it with water and add fish and a pump. It could also work for water cooling the CPU with a custom filter setup.

Anyway I was planning on some sort of a non computer object like the lamp idea lol. I have thousands of legos from when I was a kid that I kept for when ever I should have a kid. I could build a case out of the legos but don't make it like those lego mods people have done to death. Make it a shape of a car or plane or something.